On board charger vs converter

[BOBYT1630]

Ok Here's a new question. I was reading about solar panels and came across an article that said that a converter does not fully charge the batteries due to the fact that as they charge the more amps the batteries have the more resistance the put against the converter which slows the charging. so if you dry camping and charging with a generator hooked to your converter your batteries will never get a full charge because you cannot run your generator long enough to fully charge your batteries. It acts like a trickle charger.
I would like to know if I hook up an on-board charger like the have on bass boats to charge my batteries will there be any problems with the electrical circuits in my unit. as the charger I purchased has to be connected directly the batteries? The charger is a 6 amps per battery charger and will charge 2 batteries at the same time.